Ultra high molecular weight polyethylene (UHMWPE) is currently the polymer most often used in orthopaedic prostheses. This is because of the good abrasion resistance, biocompatibility and ductility exhibited by UHMWPE. Evidence indicates that UHMWPE undergoes degradation in its properties during in-vivo use and becomes prone to brittle failure. We plan to determine the specific structural properties of polyethylene important to minimizing wear, creep deformation and degradation under in-vivo environments. For this purpose we will determine the structure-property relation of various polyethylene specimens currently available for prosthesis components. Further, in order to elucidate the mechanisms and kinetics of the in-vivo degradation processes, we will subject polyethylene specimens to cyclic stresses, implant polyethylene films in animals and also implant a knee prosthesis with a polyethylene tibial plateau in rabbits, and determine the change in the structure and mechanical properties at periodic intervals. The results are expected to allow selection of materials which have the best performance for specific applications.